US11060133B2

US11060133B2 - Methods for detection and quantification of infectious carbapenem resistant Enterobacteriaceae (CRE)

Info

Publication number: US11060133B2
Application number: US16/172,379
Authority: US
Inventors: Joshua Labaer; Vel Murugan; Merica Vorachitti; Rahul Pannala; Douglas Faigel
Original assignee: Mayo Foundation for Medical Education and Research; Arizona Board of Regents of ASU
Current assignee: Mayo Foundation for Medical Education and Research; Arizona Board of Regents of ASU
Priority date: 2017-10-26
Filing date: 2018-10-26
Publication date: 2021-07-13
Also published as: US11840723B2; US20190127778A1; US20210381028A1

Abstract

Provided herein are qPCR-based methods for quantitatively detecting and differentiating between live and dead pathogenic bacteria such as carbapenem-resistant Enterobacteriaceae (CRE). Also provided herein are simple, fast, and reliable genomic DNA extraction methods with improved efficiency.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/577,359, filed Oct. 26, 2017, which is incorporated herein by reference as if set forth in its entirety.

BACKGROUND

Nosocomial infections of Carbapenem-resistant Enterobacteriaceae (CRE) have been reported in several healthcare facilities across United States. In particular, there have been multiple reports of CRE transmission through duodenoscopes in patients undergoing endoscopic retrograde cholangiopancreatography (ERCP). Treatment options for CRE nosocomial infections are limited, and invasive infections are associated with a high mortality rate. This is a substantial public health concern as approximately 500,000 ERCPs are performed annually in the US. Since the reporting of these outbreaks, the FDA has recommended additional chemical sterilization techniques such as ethylene oxide sterilization or microbiological surveillance culture, and many endoscopy units have started routinely screening patients and/or duodenoscopes for CRE. Monitoring endoscope disinfection through culture is labor-intensive, time consuming, and prone to false positives due to non-pathogenic environmental contaminants. Sterilization techniques are expensive and they are not available at most endoscopy units. Without advancements in endoscope design or reprocessing techniques, the risk of persistent contamination and transmission will continue to be a significant public health concern. Accordingly, there remains a need in the art for improved, reliable, real-time monitoring methods for detection of live pathogens on medical devices such as endoscopes.

SUMMARY OF THE DISCLOSURE

In a first aspect, provided herein is a method of detecting live and dead carbapenem-resistant Enterobacteriaceae (CRE) in a sample comprising bacteria, the method comprising: (a) incubating the sample in the presence of a DNA intercalating dye that selectively permeates into dead bacterial cells, wherein incubating comprises contacting the DNA intercalating dye to the sample for about 10 minutes and exposing the contacted sample to light for about 15 minutes to obtain a light-treated, dye-treated sample; (b) extracting genomic DNA (gDNA) from bacteria of the light-treated, dye-treated sample; (c) pre-amplifying the extracted genomic DNA, wherein pre-amplifying comprises performing 8 to 12 cycles of target-specific limited amplification using a pair of CRE-specific oligonucleotide primers selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, and SEQ ID NO:4; (d) performing a real-time PCR reaction to amplify nucleic acids of the extracted gDNA, wherein performing comprises providing the pair of CRE-specific oligonucleotide primers and a fluorescently labeled oligonucleotide probe to the extracted gDNA; and (e) determining a threshold cycle value (Ct value) and comparing the Ct value to a standard obtained by quantitative real-time PCR of known quantities of DNA of live and dead bacteria to detect live and dead bacteria in the sample. The DNA intercalating dye can be propidium monoazide (PMA™) or ethidium monoazide (EMA). Extracting gDNA can comprise contacting the sample to water and boiling the contacted sample at 100° C. for about 15 to about 25 minutes. Extracting gDNA can comprise contacting the sample to a cell lysis solution. The method can further comprise quantifying live and dead CRE detected in the sample. The sample can be a medical device-derived sample. The medical device can be a duodenoscope. The sample can be obtained by swabbing the medical device. The sample can be a patient-derived sample.

In another aspect, provided herein is a genomic DNA (gDNA) extraction method, where the method comprises resuspending a sample containing or suspected of containing bacteria in water; and boiling the resuspended sample at 100° C. for about 15 to about 25 minutes.

In a further aspect, provided herein is a method for real-time monitoring for live carbapenem-resistant Enterobacteriaceae (CRE) on a medical device, where the method comprises (a) contacting the medical device to an elution buffer comprising a DNA intercalating dye; (b) exposing the contacted medical device to light; (c) collecting the device-contacted elution buffer; (d) centrifuging the collected buffer to form a pellet of any live or dead bacteria eluted from the medical device; (e) resuspending the pellet to form a suspension; (f) extracting DNA from any live or dead bacteria in the suspension; and (g) performing quantitative real-time PCR using at least two CRE-specific oligonucleotide primers selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, and SEQ ID NO:4 and a fluorescently labeled oligonucleotide probe, wherein the presence of live CRE on the medical device is detected by comparing amplified nucleic acids to a standard curve obtained by quantitative real-time PCR of known quantities of DNA of the live CRE. The incubating step can comprise contacting the DNA intercalating dye and an enhancer to the sample. The medical device can be a duodenoscope. The sample can be obtained by swabbing the medical device.

In another aspect, provided herein is a method for rapidly screening for live carbapenem-resistant Enterobacteriaceae (CRE) in a patient sample prior to the patient being subject to an endoscopic retrograde cholangiopancreatography (ERCP), where the method comprises: (a) contacting a sample from the patient to a DNA intercalating dye; (b) exposing the contacted sample to light; (c) collecting the light exposed, contacted sample; (d) extracting genomic DNA from any bacteria in the suspension; and (e) performing quantitative real-time PCR using a set of CRE-specific oligonucleotide primers and a labeling probe, wherein the presence of live CRE is detected by comparing amplified nucleic acids to a standard curve obtaining by quantitative real-time PCR of known quantities of DNA of live CRE; wherein, if the patient sample is negative for live CRE, the duodenoscope used for the patient's ERCP is not selected for chemical sterilization, and wherein, if the patient sample is positive for live CRE, the duodenoscope is selected for chemical sterilization. The sample can be a rectal swab.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B. (A) Live cell membrane-impermeable propidium monoazide (PMA) dye selectively enters dead cells and covalently modifies DNA in the presence of bright light. (B) PMA-modified DNA cannot be amplified by qPCR.

FIG. 2 presents amplification plots demonstrating the ability to discriminate between live vs dead CRE using PMA and Enhancer. 100 μL of cultured live (100%) and heat killed (100%) CRE were incubated with either 50 μM of PMA or 50 μM PMA plus 1× concentration of enhancer for 10 minutes. Samples were exposed to light for 15 minutes before DNA extraction for qPCR analysis for NDM1. Larger Ct values reflect samples containing DNA that was not amenable for qPCR amplification due to PMA and enhancer.

FIG. 3 presents amplification plots demonstrating the ability to discriminate between live vs dead CRE using PMA and Enhancer. 100 μL of cultured live (100%) and heat killed (100%) CRE were incubated with either 50 μM of PMA or 50 μM PMA plus 1× concentration of enhancer for 10 minutes. Samples were exposed to light for 15 minutes before DNA extraction for qPCR analysis for 16S rRNA. Larger Ct values reflect samples containing DNA that was not amenable for qPCR amplification due to PMA and enhancer.

FIG. 4 demonstrates standard curves generated using known amounts of qPCR-amplified genomic DNA. C_Tvalues obtained using PMA treated and un-treated samples were used to calculate the number of molecules detected by qPCR amplification.

FIG. 5 demonstrates standard curves generated using known amounts of qPCR-amplified genomic DNA. C_Tvalues obtained using PMA treated and un-treated samples were used to calculate the number of molecules detected by qPCR amplification.

FIGS. 6 and 7 present amplification plots demonstrating efficiency of various DNA extraction methods. Smaller Ct values reflect larger amounts of DNA obtained from extraction.

FIGS. 8A-8B illustrate an exemplary serological pipet-dip test to mimic sampling from soiled or processed duodenoscopes. (A) Same number of cultured live, dead, and mixed CREs were taken in to three 15 mL conical tubes. Serological pipets were dipped in to CREs to the depth of 5 mL and retrieved to extract bacterial cells by washing pipettes in phosphate buffered saline (PBS). (B). Collected cells were exposed to PMA treatment before extracting DNA for live/dead quantification by qPCR.

FIG. 9 demonstrates quantitative detection of the number of viable CREs in biological fluids spiked with CREs at different numbers.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, and patent application was specifically and individually indicated to be incorporated by reference.

DETAILED DESCRIPTION

The methods and systems provided herein are based at least in part on the inventors' development of qPCR-based protocols for screening duodenoscopes for the presence of live or dead carbapenem-resistant Enterobacteriaceae (CRE). In particular, the inventors developed rapid qPCR-based diagnostic tests to detect, discriminate, and quantify live CRE in a sample (e.g., swab of a duodenoscope) within a few hours of sample acquisition. By integrating such protocols with an improved method for genomic DNA (gDNA) extraction, the technology described herein provides an efficient and reproducible platform to quantitatively detect and differentiate CREs and, accordingly, rapidly and reliably identify duodenoscopes that harbor carbapenem-resistant Enterobacteriaceae (CRE) and pose a risk to subsequent endoscopic retrograde cholangiopancreatography (ERCP) patients.

There is no standard qPCR-based technology to detect and differentiate live versus dead CREs in clinical specimens. Currently existing qPCR assays detect CREs but do not distinguishes the detected nucleic acids from “leftover” DNA of dead cells in the specimen. Our technology identifies live, potentially harmful bacteria that can lead to unwanted nosocomial infections. Advantages of the methods and systems provided herein include the ability to produce reliable, reproducible results within about 2-3 hours of sampling and to enrich, detect, and quantify only live pathogens in a sample. The methods and systems provided herein will be particularly beneficial to institutions or clinical practices that do not have ready access to reference laboratories or chemical sterilization facilities. Without being bound to any particular theory or mechanism of action, it is believed that DNA intercalating agents like propidium monoazide (PMA™, Biotium) can selectively permeate dead cells and render their DNA unusable for polymerase chain reaction (PCR) amplification, thereby selectively enriching for quantitative polymerase chain reaction (qPCR)-capable DNA template from living pathogens that can be quantified using a real-time PCR assay such as, for example, a TaqMan® (Applied Biosystems™) gene expression assay.

In a first aspect, provided herein is a method for detecting about 5 to about 9 logs of a pathogen (e.g., pathogenic bacteria) in a sample. In certain embodiments, the method to detect and quantify live and dead bacteria of a particular category in a sample comprising bacteria comprises: (a) incubating the sample in question with a DNA intercalating dye that selectively permeates into dead bacterial cells, wherein incubating comprises contacting the DNA intercalating dye to the gDNA and exposing the contacted gDNA to light for about 15 minutes to obtain a light-treated gDNA/dye sample; (b) extracting genomic DNA (gDNA) from bacteria of the sample; (c) pre-amplifying the extracted genomic DNA wherein pre-amplifying comprises 8-12 cycles of target-specific limited amplification using at least two pathogen-specific oligonucleotide primers; (e) performing a real-time PCR reaction to amplify nucleic acids of the light-exposed gDNA, wherein performing comprises providing the pair of pathogen-specific oligonucleotide primers and a labeling probe to the light-treated gDNA/dye sample; and (f) determining a threshold cycle value (Ct value) and comparing the Ct value to a standard obtained by quantitative real-time PCR of known quantities of DNA of the live and dead bacteria. In some cases, the pathogenic bacteria are CRE and the least two primers are CRE-specific oligonucleotide primers selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, and SEQ ID NO:4. In certain embodiments, PreAmp MasterMix (product of Applied Biosystems, Inc.) is used for the pre-amplifying step.

Real-time PCR can amplify DNA or the like to several hundred thousand-fold by continuously performing amplification cycles of “thermal denaturation→annealing with a primer→polymerase extension reaction.” Real-time PCR monitors the resulting amplification products in real time to perform a quantitative assay of the trace nucleic acids. In certain embodiments, the gene expression assay is a “TaqMan® probe” method (product of Applied Biosystems, Inc.), which uses as a probe an oligonucleotide modified at the 5′ terminal with a fluorescent material and at the 3′ terminal with a quencher material. TaqMan® PCR is a method of detecting and quantifying the presence of a DNA sequence. Briefly, a FRET oligonucleotide probe is designed that it overlaps a target sequence of interest. The FRET probe and PCR primers (at least one upstream and at least one downstream of the target sequence of interest) are cycled in the presence of a thermostable polymerase and dNTPs.

In some cases, the method further comprises a pre-amplification step to increase the limit of detection (sensitivity). For example, 8 to 12 pre-amplification cycles can be added to increase the sensitivity of qPCR-based assays that are not sufficiently sensitive to detect at least 1-10 live bacteria.

In some cases, the DNA intercalating dye (also known as a nucleic acid intercalating moiety) is contacted to the sample for incubated for sufficient time for the dye to non-covalently bind and intercalate in the nucleic acid. For example, the dye and nucleic acid may be incubated for at least 5 minutes and usually not more than about 2 hours, where complex formation will normally be complete in less than about one hour, usually in fewer than 30 minutes at room temperature. In some cases, the DNA intercalating dye is contacted to the sample for a length of time between about 5 minutes to about 120 minutes (e.g., about 5, 10, 15, 20, 25, 30 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 150, 120 minutes, inclusive). Preferably, the DNA intercalating dye is contacted to the sample for about 5 to about 30 minutes. More preferably, the DNA intercalating dye is contacted to the sample for about 10 minutes. In some cases, contacting of the sample and DNA intercalating dye are brought together in an appropriately buffered medium.

The term “labeling probe” generally, according to various embodiments, refers to a molecule used in an amplification reaction, typically for quantitative or qPCR analysis, as well as end-point analysis. Such labeling probes may be used to monitor the amplification of the target polynucleotide. In some embodiments, fluorescently labeled oligonucleotide probes present in an amplification reaction are suitable for monitoring the amount of amplicon(s) produced as a function of time. Such fluorescently labeled oligonucleotide probes include, but are not limited to, the 5′-exonuclease assay TaqMan® labeling probes described herein (see also U.S. Pat. No. 5,538,848). While TaqMan® assays and reagents are used to demonstrate the features of embodiments of the methods described herein, one of ordinary skill in the art will recognize that a variety of assays including probe reagents may be utilized to produce data that may be analyzed according to various embodiments of the methods provided herein.

In some cases, the method also comprises, as a first step, performing PCR using as templates serially-diluted standard samples the concentrations of which are known. Numbers of cycles (threshold cycles: Ct values) required to each a certain constant amount of amplification product are then determined. A calibration line is then prepared by plotting these Ct values and initial DNA amounts along the abscissa and the ordinate, respectively. The amount of target DNA in the sample can be determined using this Ct value and the above-mentioned calibration line. In a real-time PCR reaction, dead cell DNA will show delayed amplification and higher Ct than live cells. In a mixed population, qPCR permits quantitation of viable bacteria in the sample.

Preferably, the DNA intercalating dye is cell membrane-impermeable and thus can be used to selectively modify DNA from dead cells with compromised membrane integrity, while leaving DNA from viable cells intact. In some cases, the DNA intercalating agent is PMA™ (Biotium). PMA™ is a high affinity photoreactive DNA binding dye useful in the selective detection of viable pathogenic cells by quantitative real-time PCR. In some cases, genomic DNA is contacted to PMA™, or to PMA plus a PMA™ enhancer, which improves PMA™- and PMAxx™ (Biotium)-mediated discrimination between live and dead gram-negative bacteria. Referring to FIGS. 1A and 1B, when a double-stranded DNA-PMA complex is photoactivated by exposure to light (e.g., visible light), the monoazide group of PMA is converted to a highly reactive nitrene radical which readily reacts with any hydrocarbon moiety to form a stable covalent bond (see FIG. 1B), and prevents its use as a template for PCR. In other cases, the DNA intercalating dye is ethidium monoazide (EMA). Preferably, photoactivation of the DNA intercalating dye occurs open exposure to one or more sources of light (e.g., visible light) which emits light having a wavelength ranging from 350 to 1100 nm. Appropriate light sources include, without limitation, a light emitting diode (LED), laser, incandescent lamp, and halogen light. In some cases, the light source is one or more LEDs having an output wavelength of about 465 nm to about 475 nm.

In some cases, it may be advantageous for the methods of this disclosure to include a centrifugation step. For example, it may be advantageous to use centrifugation to enrich for live and dead pathogens when screening a sample for the presence of pathogens. Live and dead bacteria can be enriched by centrifugation at 5000×g for about 5 to about 20 minutes. In some case, centrifugation is at 5000×g for about 10 to about 15 minutes. Pelleted bacteria can be lysed in an appropriate buffer to facilitate DNA extraction. In some cases, extracted DNA will be subjected to qPCR amplification to quantify the number of live bacteria from the sample.

As used herein the term “pathogen” refers to an organism, including a microorganism, which causes disease in another organism (e.g., animals and plants) by directly infecting the other organism, or by producing agents that causes disease in another organism (e.g., bacteria that produce pathogenic toxins and the like). As used herein, pathogens include, but are not limited to bacteria, protozoa, fungi, parasites, viruses, or any combination thereof, wherein each pathogen is capable, either by itself or in concert with another pathogen, of eliciting disease in vertebrates including but not limited to mammals, and including but not limited to humans. As used herein, the term “pathogen” also encompasses microorganisms which may not ordinarily be pathogenic in a non-immunocompromised host. In some cases, pathogens detected by the method comprise one or more clinically relevant species of bacteria, virus, fungus, mycoplasma, or parasite.

In some cases, the pathogens are gram negative bacteria, a group which encompasses cocci, nonsporulating rods, and sporulating rods. Gram-negative bacteria include without limitation Enterobacteriaceae, Carbapenem-resistant Enterobacteriaceae (CRE), Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Burkholderia cepacia, Alcaligenes xylosoxidans, Acinetobacter, Haemophilus, Neisseria species, Francisella tularensis, Yersinia pestis, Burkholderia pseudomallei, Burkholderia mallei, Rickettsia prowazekii, Coxiella burnetti, Campylobacter jejuni, Shigella, Moraxella catarrhalis, and Chlamydia trachomatis. In certain embodiments, the methods detect Carbapenem-resistant Enterobacteriaceae (CRE) in a sample.

In certain embodiments, the method comprises subjecting a PMA- and light-treated sample to 5′ nuclease-based real-time PCR amplification using pathogenic specific oligonucleotide primers and a qPCR probe.

Samples appropriate for the methods provided herein include clinical specimens (diagnostic samples collected as part of standard clinical procedures) and samples obtained by swabbing or otherwise sampling a medical device.

In another aspect, provided herein is a system that integrates the above-described elements for quantitative detection and differentiation of live versus dead CRE in a sample.

It will be advantageous in some embodiments to apply Clinical Laboratory Standards Institute (CLSI) guidelines (EP17-A2) to calculate Limit of Blank (LoB), Limit of Detection (LoD) and Limit of Quantification (LoQ) for each assay and to determine false positive and false negative rates for each of the assays within the detectable range. LoB is the highest measurement result that is likely to be observed for a blank. LoD is the lowest number of DNA copies that can be detected, perhaps not quantified. LoQ is the lowest number of DNA copies in the sample that can be quantified with acceptable precision under the stated experimental condition. Determining the LOB, LoD, and LoQ helps to define the detection capabilities and limitations of a particular assay.

In a further aspect, provided herein is a simple, fast, and reliable genomic DNA (gDNA) extraction method. In some embodiments of the extraction method, cells are re-suspended in water and boiled at 100° C. for about 20-30 minutes. No other processing required before quantification. As described herein and the Examples that follow, the gDNA extraction methods facilitate maximum DNA extraction with >90% efficiency relative to commercially available DNA extraction methods.

In a further aspect, provided herein is a method for real-time monitoring for live pathogenic bacteria on a medical device (e.g., duodenoscope), wherein the method comprises (a) contacting the medical device to an elution buffer comprising a DNA intercalating dye; (b) exposing the contacted medical device to light; (c) collecting the device-contacted elution buffer; (d) centrifuging the collected buffer to form a pellet of any live or dead bacteria eluted from the medical device; (e) resuspending the pellet to form a suspension; (f) adding a cell lysis solution to the suspension to extract DNA from any lysed live or dead bacteria in the suspension; and (g) performing quantitative real-time PCR using a set of pathogen-specific oligonucleotide primers and a labeling probe, wherein the presence of live pathogenic bacteria on the medical device is detected by comparing amplified nucleic acids to a standard curve obtained by quantitative real-time PCR of known quantities of DNA of the pathogenic bacteria. An exemplary sampling protocol is illustrated in FIGS. 8A-8B. In some cases, the live pathogenic bacteria comprise one or more clinically relevant species such as Carbapenem-resistant Enterobacteriaceae (CRE) or another gram-negative bacteria species. The sample can be obtained by swabbing the medical device. In some cases, the medical device is a duodenoscope. In such cases, sample collection can be performed according to the Federal Drug Administration's interim duodenoscope sampling method guidelines or a variation of such guidelines. Samples that are important to obtain include, without limitation, the Instrument Channel Flush and submerged distal end and elevator mechanism sample. In some cases, the elution buffer is the FDA recommended elution buffer: phosphate buffered saline (PBS) with 0.02% Tween-20.

In another aspect, provided herein is a method for rapidly screening for live Carbapenem-resistant Enterobacteriaceae (CRE) in a patient sample prior to the patient being subject to an endoscopic retrograde cholangiopancreatography (ERCP). The method can comprise (a) contacting a sample from the patient to a DNA intercalating dye; (b) exposing the contacted sample to light; (c) collecting the light exposed, contacted sample; (d) adding a cell lysis solution to the sample of (c) to extract DNA from any lysed live or dead bacteria in the suspension; and (e) performing quantitative real-time PCR using a set of CRE-specific oligonucleotide primers and a labeling probe, wherein the presence of live CRE is detected by comparing amplified nucleic acids to a standard curve obtaining by quantitative real-time PCR of known quantities of DNA of live CRE; wherein, if the patient sample is negative for live CRE, the duodenoscope used for the patient's ERCP is not selected for chemical sterilization, and wherein, if the patient sample is positive for live CRE, the duodenoscope is selected for chemical sterilization. In some cases, the sample is a rectal swab.

In another aspect, provided herein is an article of manufacture such as a kit comprising components useful for quantitative detection and differentiation of live versus dead CRE in a sample according to the methods provided herein. In some cases, the kit comprises one or more of a DNA intercalating dye, pathogen-specific primers, a lysis buffer, sampling materials (e.g., swabs, elution buffer), and instructions for using the same to detect and differentiate, and quantify live versus dead CRE in a sample.

So that the methods and systems provided herein may more readily be understood, certain terms are defined:

Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Any reference to “or” herein is intended to encompass “and/or” unless otherwise stated.

As used herein, “about” means within 5% of a stated concentration range, density, temperature, or time frame.

As used herein, the term “amplified product” refers to polynucleotides that are copies of a particular polynucleotide, produced in an amplification reaction. An “amplified product,” according to the invention, may be DNA or RNA, and it may be double-stranded or single-stranded. An amplified product is also referred to herein as an “amplicon”.

As used herein, “subject” means any individual, which is a human, a non-human animal, any mammal, or any vertebrate. The term is interchangeable with “patient,” “recipient” and “donor.” A subject can be any mammalian species alive or dead.

Although the embodiments are described in considerable detail with reference to certain methods and materials, one skilled in the art will appreciate that the disclosure herein can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein.

EXAMPLES

Nosocomial infections with CRE have been reported in several healthcare facilities across United States. Treatment options are limited and invasive infections are associated with a high mortality rate. In United States, there have been multiple reports of CRE transmission through duodenoscopes in patients undergoing endoscopic retrograde cholangiopancreatography (ERCP). This is a substantial public health concern as approximately 500,000 ERCPs are performed annually in the United States. Since the reporting of these outbreaks, many endoscopy units have started routinely screening patients and/or duodenoscopes for CRE. Currently at Mayo Clinic Arizona (MCA), a rectal swab is obtained on all patients and sent to the MCR reference laboratory for a polymerase chain reaction (PCR)-based test. Turnaround time for results of this assay is usually 48-72 hours. Duodenoscopes are sequestered for this time, which places a substantial time and financial burden on the clinic. Clinical laboratories do not perform duodenoscope cultures. Furthermore, cultures from reprocessed duodenoscopes do not detect all bacteria and any viruses or parasites. In discussion with clinicians at MCA, we identified developing and validating real-time monitoring methods to assess the risk of infection as a critical unmet need.

PCR amplification from b/a_KPC, bla_NDM-1 and 16sRNA has been previously reported to detect CRE in clinical samples. Based on literature, we selected three independent target sequences of 16S rRNA, Klebsiella pneumonia carbapenemenase (KPC), and New Delhi metallo β-lactamase (NDM1) to design and order custom TaqMan assays from Applied Biosystems. We ordered three TaqMan assays each for NDM1 and KPC, and ordered one TaqMan assay using a previously published sequence for CRE 16S rRNA. All TaqMan assays were tested for their ability to amplify using 8-logs of genomic DNA (˜2×10⁹to <1 molecules) as a starting template. Primer sequences are presented in Table 1. Our process included 10 cycles preamplification of templates using PreAmplification MasterMix from Thermo Fisher Scientific. Preamplification improves the sensitivity of detection and provides extremely high correlation between amplified and unamplified cDNA. The best performing assay was selected based on its ability to 1) Perform linear PCR reaction over seven-log range of input DNA template using eight to 10 fold dilutions (when Ct is plotted against log₁₀input template quantity, R2 value≥0.98); 2) Detect≥10 copies of the target; 3) Exhibit 100%±10% amplification efficiency when tested over five orders of magnitude of input template; 4) No detectable PCR amplification (Ct value>37) when there is no template added (No template control-NTC) (Table 2). NDM1 and KPC1 assays were able to detect>10 copies of genomic DNA compared to 16S rRNA probe was able to detect<1 copies of genomic DNA. It is important to note that multiple copies of 16S rRNA sequences are present within the genome.

We used TaqMan™ assay chemistry for the detection of PCR products. We note that the TaqMan™ assay is proprietary for Thermo Fisher Scientific, so the actual TaqMan™ probe sequence is unknown.

TABLE 1

Primer sequences

Gene		Forward Primer	Reverse Primer
Name	Assay ID	(5′-3′)	(5′-3′)

NDM1	AIFATXV	GACAAGATGGGCGGTATG	CTGGTTCGACAACGCATTGG
		GA (SEQ ID NO: 1)	(SEQ ID NO: 2)

KPC1	AID1VRN	CGCCGCCAATTTGTTGCT	GCCGATAGAGCGCATGAAG
		(SEQ ID NO: 3)	(SEQ ID NO: 4)

16S	AILJKSZ	TGGAGCATGTGGTTTAAT	TGCGGGACTTAACCCAACA
		TCGA (SEQ ID NO:	(SEQ ID NO: 6)
		5)

After exploring many different DNA extraction methods, we modified an in-vitro diagnostics (IVD)-approved DNA extraction method recommended by Qiagen (QIAamp DSP DNA Mini Kit, IVD; Cat#61304) to develop a method for specific extraction of DNA from CRE. This process takes >30 minutes of DNA extraction time.

Detection and Quantification of Live Vs Dead CRE:

Detection and discrimination of live and dead CRE is based on our hypothesis that DNA intercalating agents like propidium monoazide (PMA) can selectively permeate dead cells and rendering their DNA inept for PCR amplification, thereby selectively enriching a live, qPCR-capable DNA template that can be quantified using TaqMan™ assays (FIG. 1). We varied PMA incubation time and concentration, which is extremely important for differentiating live and dead bacteria by using live and heat killed CRE. The amount of time and temperature necessary to generate heat-killed bacteria that yields PCR quantifiable genomic DNA was also modified to be <30 min at 100° C. PMA concentration and incubation time was selected based on maximum Ct difference that is observed between live and heat killed CRE. PMA concentration at 50 μM, with 15 minute exposure to light showed maximum difference in Ct values. We included an addition of enhancer solution that helps in differentiating live versus dead cells. PMA enhancer for gram negative bacteria was designed to improve PMA-mediated discrimination between live and dead gram-negative bacteria. When CRE bacterial DNA is amplified by PCR, samples pre-treated with Enhancer show a decrease in the signal from dead cells, with no change in the signal from live cells.

After modifying the concentration and incubation time of PMA and Biotium PMA enhancer, we tested the ability to detect and quantify the number of CRE (FIG. 2, FIG. 3, and Table 3).

Live and heat killed CRE showed 9 to 12 Ct difference for 16S and 6 to 7 Ct difference for NDM1. We calculated the number of molecules based on standard curve generated by plotting Ct values against the number of molecules (FIG. 4, FIG. 5). Heat killed CRE showed >99.5% reduction in the number of PCR amenable genomic DNA molecules (Table 3; Live vs Dead (with PMA and enhancer)). The reduction that we saw was specifically due to PMA treatment (Table 4; PMA vs no PMA (with enhancer)). Enhancer significantly increases the effect of PMA.

To further improve DNA extraction methods for faster, cost-effective and reproducible generation of qPCR quality DNA, we tested various DNA extraction methods. We tested Qiagen DNA extraction kit, PAXGene DNA extraction kit, PAXGene RNA extraction kit and Trizol reagents. Qiagen DNA extraction method outperformed all other methods in optimum DNA yield (Table 5 and FIGS. 6-7).

We also tested a boiling DNA extraction method and compared it to other commercially available DNA extraction methods. Our boiling DNA extraction method yielded more DNA compared to all commercial DNA extraction methods that we tested (Table 6).

Next we explored DNA extraction efficiency using different numbers of cells and reproducibility using triplicates. DNA extraction method is reproducible and works across different concentrations of cells (Table 7).

By spiking biological fluids with CREs at different numbers, we were able to quantitatively detect the number of viable CREs using our assay. See FIG. 9. This demonstrates our ability to detect viable CREs in biological fluids without any PCR inhibition.

These data demonstrate our development of an improved, reproducible DNA extraction method that yields high quality DNA using an IVD-approved DNA extraction kit to detect and discriminate live and dead CRE. Here we show that when the same number of live and dead bacteria are quantified using our qPCR-based methods, 1.5% of the dead bacteria are detected as compared to the number of live bacteria, where we detect 100%. This suggests that if we have 100 live bacteria, we will detect all of them, but if we have 100 dead bacteria we will detect them (collectively) as 1.5 live bacteria. We are working to improve the detection of dead bacteria to “0” when there is no live bacteria in a population of dead bacteria.

In summary, duodenoscope-related CRE infections are a major public health concern in the United States. It is unlikely that there will be rapid advancements in endoscope design or reprocessing techniques to eliminate the risk of persistent contamination and possible transmission. Development of the reliable, real-time monitoring methods of this disclosure to detect live pathogens on endoscopes will have direct and immediate clinical applicability. The methods described herein have the potential to decrease resource utilization and improve efficiency by identifying CRE-contaminated duodenoscopes and selectively targeting them for sterilization.

TABLE 2

TaqMan Assay Performance and Selection

		PCR		NTC
		Amplifi-	Lower Detection	Amplifi-
		cation	limit (# of	cation
Tar-		efficiency	molecules of	(Ct
get	Assay ID	(%)	genomic DNA)	Value > 37)

NDM1	AICSXLF	93	>10	Passed
	AIFATXV**	96	>10	Passed
	AIHSQAB	95	>10	Passed
	AIKAMMR	95	>10	Passed
KPC1	AIBJZE7	89	>10	Passed
	AID1VRN**	91	>10	Passed
	AIGJR33	88	>10	Passed
	AII10GJ	87	>10	Passed
16S*	AILJKSZ**	100 and 104	<1	Passed

*Assay tested using two different strains of CRE (NDM1⁺ KPC⁻ and NDM1⁻ KPC⁺).
**Selected assays

TABLE 3

qPCR differentiation of live vs dead CRE

Live
or		16S	NDM1

Dead	Treatment	Ct Mean	Difference	Ct Mean	Difference

Live vs Dead (with PMA and enhancer)

Live	Bacteria +	10.41	12.40	13.59	6.68
	Enhancer + PMA
Dead	Heat Bacteria +	22.81		20.27
	Enhancer + PMA

PMA vs No P VIA (with enhancer)

Dead	Heat Bacteria +	22.81	12.42	20.27	7.33
	Enhancer + PMA
Dead	Heat Bacteria +	10.39		12.94
	Enhancer

Enhancer vs No Enhancer (with PMA)

Dead	Heat Bacteria +	22.81	9.66	20.27	6.04
	Enhancer + PMA
Dead	Heat Bacteria +	13.15		14.23
	PMA

TABLE 4

The number of genomic DNA molecules that are not
suitable for qPCR by PMA and enhancer treatment

16S Assay

NDM1 Assay

		# of	%	# of	%
Live		genomic	reduction	genomic	reduction
or		DNA	by	DNA	by
Dead	Treatment	Molecules	treatment	Molecules	treatment

Live vs Dead (with PMA and enhancer)

Live	Bacteria +	1419960	99.94%	1344610	99.53%
	Enhancer + PMA
Dead	Heat killed	900		6339
	bacteria +
	Enhancer + PMA

PMA vs No PMA (with enhancer)

Dead	Heat killed	900	99.94%	6339	99.75%
	bacteria +
	Enhancer + PMA
Dead	Heat killed	1443953		2586418
	bacteria +
	Enhancer

Enhancer vs No Enhancer (with PMA)

Dead	Heat killed	900	99.43%	6339	99.12%
	bacteria +
	Enhancer + PMA
Dead	Heat killed	158309		721967
	bacteria +
	PMA

TABLE 5

DNA extraction using different commercially
available DNA extraction methods

16S

NDM1

	Average		Average
Methods Used	Ct Mean	Ct SD	Ct Mean	Ct SD

Qiagen DNA Extraction	22.35	0.08	15.32	0.11
Method
PAXGene DNA Extraction	20.91	0.03	12.90	0.06
Kit
PAXGene RNA Extraction	24.57	0.07	19.23	0.20
Kit
Trizol Reagent	24.93	0.02	20.60	0.20
No template Control	25.21	0.00	30.52	3.02

TABLE 6

CPD designed boiling DNA extraction method

DNA

16S

NDM1

extraction	Average		Average
Method	Ct Mean	Ct SD	Ct Mean	Ct SD

Qiagen	21.7	4.1	26.0	5.1
PAXGene DNA	24.5	0.7	31.9	5.1
Puregene	23.2	2.3	28.7	4.9
Boiling Prep	21.2	3.6	25.3	6.3
NTC	24.4	1.3	ND	ND

ND = Not detectable

TABLE 7

CPD designed boiling DNA extraction method works across different
concentrations of cells

Triplicate

16S

NDM1

cell	Mean		Average		Mean		Average
numbers	Ct	SD	Ct	SD	Ct	SD	Ct	SD

10K	20.26	0.015	19.88	0.32	22.15	0.026	22.05	0.09
10K	19.9	0.116			21.94	0.029
10K	19.47	0.005			22.07	0.005
1K	23.58	0.6	23.23	0.37	25.68	0.248	25.21	0.47
1K	22.73	0.039			24.57	0.025
1K	23.39	0.352			25.39	0.101
100	23.43	0.061	23.7	0.27	27.56	0.081	27.18	0.39
100	24.07	0.385			27.34	0.008
100	23.61	0.028			26.64	0.005
PBS	ND	ND	ND	ND	ND	ND	ND	ND

ND = Not detectable amplification

Claims

We claim:

1. A method of detecting live and dead carbapenem-resistant Enterobacteriaceae (CRE) in a sample comprising bacteria, the method comprising:

(a) incubating the sample in the presence of a DNA intercalating dye that selectively permeates into dead bacterial cells, wherein incubating comprises contacting the DNA intercalating dye to the sample for about 10 minutes and exposing the contacted sample to light for about 15 minutes to obtain a light-treated, dye-treated sample;

(b) extracting genomic DNA (gDNA) from bacteria of the light-treated, dye-treated sample;

(c) pre-amplifying the extracted genomic DNA, wherein pre-amplifying comprises performing 8 to 12 cycles of target-specific limited amplification using at least two CRE-specific oligonucleotide primers selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, and SEQ ID NO:4 to produce a pre-amplified product;

(d) performing a real-time PCR reaction to amplify the pre-amplified product, wherein performing comprises providing the at least two CRE-specific oligonucleotide primers and a probe detectably labeled with a fluorescent dye and a quencher and configured for the real-time PCR reaction using the at least two CRE-specific oligonucleotide primers; and

(e) determining a threshold cycle value (Ct value) and comparing the Ct value to a standard obtained by quantitative real-time PCR of known quantities of DNA of live and dead bacteria to detect the presence of live and dead carbapenem-resistant Enterobacteriaceae (CRE) in the sample.

2. The method of claim 1, wherein the DNA intercalating dye is propidium monoazide (PMA) or ethidium monoazide (EMA).

3. The method claim 1, wherein the incubating step comprises contacting the DNA intercalating dye and an enhancer to the sample.

4. The method of claim 1, wherein extracting gDNA comprises contacting the sample to water and boiling the contacted sample at 100° C. for about 15 to about 25 minutes.

5. The method of claim 1, wherein extracting gDNA comprises contacting the sample to a cell lysis solution.

6. The method of claim 1, further comprising quantifying live and dead CRE detected in the sample.

7. The method of claim 1, wherein the sample is a medical device-derived sample.

8. The method of claim 7, wherein the medical device is a duodenoscope.

9. The method of claim 7, wherein the sample is obtained by swabbing the medical device; and contacting the swab to an elution buffer comprising the DNA intercalating dye.

10. The method of claim 1, wherein the sample is a patient-derived sample.

11. A method for rapidly screening for live carbapenem-resistant Enterobacteriaceae (CRE) in a patient sample prior to the patient being subjected to endoscopic retrograde cholangiopancreatography (ERCP), wherein the method comprises:

(a) contacting a sample from the patient to a DNA intercalating dye comprising adding the patient sample to a buffer comprising the dye;

(b) exposing the contacted sample to light;

(c) collecting the light exposed, contacted sample;

(d) extracting genomic DNA from any bacteria in the sample; and

(e) performing quantitative real-time PCR using a set of CRE-specific oligonucleotide primers and a probe detectably labeled with a fluorescent dye and a quencher and configured for the real-time PCR reaction using the at least two CRE-specific oligonucleotide primers, wherein the presence of live CRE is detected by comparing amplified nucleic acids to a standard curve obtaining by quantitative real-time PCR of known quantities of DNA of live CRE; wherein, if the patient sample is negative for live CRE, the duodenoscope used for the patient's ERCP is not selected for chemical sterilization, and wherein, if the patient sample is positive for live CRE, the duodenoscope is selected for chemical sterilization.

12. The method of claim 11, wherein the sample is a rectal swab.